It has been known for a long time that it is possible to improve the corrosion resistance of the said surfaces by subjecting them, after zinc plating, to a passivation treatment using chromate-plating baths. However, for components used under an engine bonnet and therefore subjected during use to thermal shocks and to corrosion, the requirements of the automobile industry have recently become more stringent since manufacturers now want the corrosion resistance to be in accordance after the said surfaces have been subjected to a thermal shock for one hour at 120.degree. C. or even 180.degree. C.
The corrosion resistance is assessed in the laboratory by an accelerated Salt Spray Test carried out according to the AFNOR NFX41002/ASTM B117 73/DIN40046-11 standard. The automobile industry wants components which have been subjected to a thermal shock to show no zinc salt (or white rust) after exposure to the salt spray for 200 hours and no red rust after 400 hours or indeed 600 hours. Moreover, also after thermal shock, the said components should also successfully undergo natural-corrosion tests.
Moreover, components intended for the automobile industry should satisfy aesthetic color requirements: yellow, green and more particularly black. This black color, which should be deep, glossy and uniform, is particularly difficult to obtain when, at the same time, a high corrosion resistance is demanded after thermal shock for one hour at 120.degree. C.
Chromate-plating baths based on chromium, copper and silver or molybdenum which make it possible to obtain a shiny black coating are described in Patent FR 2,522,023. However, after thermal shock for one hour at 120.degree. C., this type of coating does not last longer than 48 hours in the corrosion test with salt spray.
The use of a chromate-plating bath comprising an acrylic polymer and a phosphate makes it possible, according to Patent EP 264 472, to obtain a beautiful black coloring which is resistant to corrosion in salt spray after thermal shock. However, it is more difficult to bring such a bath into operation than a conventional chromate-plating bath. Moreover, the film obtained proves to be unstable (formation of yellow chromium salts) and the use of a polymer leads to the emergence of "drops" on the components treated on jigs and to difficulties in cleaning the equipment (centrifugal drier Jigs) used in the treatment.
It has also been proposed to subject the components to a conventional chromate plating and then to treat them in a separate bath containing the acrylic polymer and the phosphate. This two-stage process does not give reproducible results and, like the above process, has the disadvantage of leading to the formation of "drops".
It has now been found that, by using a chromate-plating bath comprising cobalt instead of copper and a top coat bath based on colloidal silica, it is possible to obtain, on zinc, zinc alloy or cadmium surfaces, a coating with a beautiful black colouring exhibitions excellent corrosion resistance in salt spray after thermal shock.